The invention relates to an impression roller for use in electrostatically assisted printing.
In this method of printing, the transfer of ink from a printing design cylinder to a web of material such as paper, paperboard, fabric or plastic film, is assisted by the electrostatic attraction of ink to the web. This is accomplished by passing the web through a nip region, where the web is contacted on its topside by the impression roller and on its underside by the printing design roller. A voltage is applied to the impression roller and current is conducted through it to the area of the nip region. There, a voltage is generated between the impression roller and the printing design roller which is at ground potential, and an electrical charge is distributed across the area of the web contacting the two rollers.
Impression rollers have generally been of two types: a two-layer roller and a three-layer roller. In the two-layer roller, an insulated core is covered with an outer layer of semiconductive material. A voltage is applied to the outer layer of semiconductive material at the top of the roller, so that current travels through the semiconductive layer to reach the nip region at the bottom of the roller. In the three-layer roller, a layer of material considerably more conductive than the semiconductive material is disposed around the insulated core, but beneath the outer surface and the semiconductive layer. When a voltage is applied across the top of the impression roller, current travels through the semiconductive material to the relatively more conductive layer. It then bypasses a great portion of the semiconductive layer as it travels around the circumference of the roller to the nip region. This results in the three-layer roller having considerably less resistance than a two-layer roller with a comparable semiconductive layer.
Printing applications are numerous and varied, and the owner of an electrostatically assisted printing machine may desire to print images on webs of varying widths. In some instances the webs may be considerably narrower in width than the length of the rollers over which it travels. In those instances there is a great deal of excess electrical energy dissipated in areas of the impression roller that extend beyond the web.
To remedy this, a voltage has been applied through a group of stainless steel sliding blades. The blades outside the width of web are lifted to a disengaging position to shorten the effective width over which voltage is applied to the impression roller. This system, however, is not apparently suitable for use with a three-layer roller, because substantial current would travel to the ends of the roller as well as around its circumference, due to the charge-distributing function of the conductive layer.